This invention relates to optical waveguides tuned by thermo-optic control.
The ability to effect localized changes in refractive index (xcex7) in planar lightguide circuits (PLC s) is an adjustment tool that is useful in a variety of devices. For example, this mechanism can be used to tune the phase in Mach-Zehnder Interferometers (MZI). See e.g. B. Zheng and C. Zheng, xe2x80x9cA study of directional coupler modulator with thermo-controlled coupling coefficient, Integrated Optoelectronics, Beijing, China: SPIE Vol. 2891, 1996, pp. 178-182. Directional couplers having a 50% power splitting ratio are needed in Mach-Zehnder Interferometers (MZI). Directional couplers where coupling ratios vary from zero to one are useful in Fourier filters and ring filters. Couplers are widely used in optical transmission systems.
In these applications, the refractive index in the device as fabricated must be very close to the design value so that the waveguide has the desired spectral response. An important evolution for silica based PLCs is toward more compact devices, which require a higher refractive index difference (xcex94) between the core and cladding. A disadvantage of using high (xcex94) is that fabrication variations in refractive index increase. In tunable couplers, normal fabrication tolerances cause variations in the coupling ratios of xc2x15% for 3 dB couplers in silica with low xcex94s (xe2x89xa60.7%). A method for actively tuning the refractive index in local regions of a planar waveguide after fabrication would be beneficial by: 1) increasing device yield, and 2) providing additional functionality by allowing the refractive index in a local region of the waveguide to be modulated.
For silica waveguides the most effective post-fabrication technique to actively tune the PLC is to use thermal heaters and rely on the thermal dependence of the refractive index (dxcex7/dT). Heaters can be deposited on the surface of the waveguide and used to tune the refractive index. However, the refractive index of silica is relatively insensitive to temperature, thus requiring a substantial temperature change to effect the desired adjustment. This requires both higher power and longer heating times. This is especially the case where the device structure has high thermal inertia between the heater and the core of the waveguide, and where the heater and the waveguide are not well aligned. A xcex7 tunable waveguide with more sensitive and more rapid thermal response would represent a significant advance in the art.
We have developed a device structure and fabrication technique for improving the thermal efficiency of tunable waveguide heaters, thereby yielding devices with improved tuning sensitivity and response. The technique involves forming the heater strip on the upper cladding of a dual clad planar waveguide, then using the heater as a mask for forming a heat channel between the heater and the waveguide core. In an alternative embodiment, the heater strip can also used to define the core itself.